Rate coefficients of H-atom abstraction from ethers and isomerization of alkoxyalkylperoxy radicals

Atmospheric insight into the reaction mechanism and kinetics of isopropenyl methyl ether (i-PME) initiated by OH radicals and subsequent oxidation of product radicals

Studies on primary gas-phase reactions of emitted saturated and unsaturated ethers with oxidants and subsequent secondary reactions of product radicals with O₂ in the presence of NO are important in their atmospheric chemical processes. To accomplish these findings, we have examined the chemistry of OH-initiated oxidation of isopropenyl methyl ether (i-PME) CH₃C(CH₂)OCH₃ by electronic structure ca using density functional theory. Our energetic calculations show that OH additions to carbon-carbon double bonds of i-PME are more favorable reaction pathways than H-abstraction reactions from the various CH sites of the titled molecule. The rate constant values which are obtained from the transition state theory also signify that OH-addition reactions have faster reaction rates than H-abstraction reactions. Our calculated total rate constant of the reaction is found 9.90 × 10^-11 cm³ molecule^-1 s^-1. The percentage branching ratio calculations imply that OH-addition reactions have 98.09% contribution in the total rate constant. The atmospheric lifetime of i-PME is found to be 2.8 h. Further, we have identified 2-hydroxy-2-methoxypropanol, methyl acetate, methy-1,2-hydroxyacetate and 1-hydroxypropane-2-one, 1,2-dihydroxypropan-2-yl format, 2-hydroxyacetic acid, acetic acid, and formaldehyde from the secondary oxidation of product radicals.

Iodine-mediated formal [3 + 2] annulation for synthesis of furocoumarin from oxime esters

A novel synthesis of furocoumarins was developed by a reaction between oxime esters and 4-hydroxycoumarins. The reaction was proposed to undergo radical mechanism mediated by iodine, a cheap and common laboratory reagent. Mechanistic studies showed the key for the successful transformation was the presence of α-iodoimine intermediate which facilitated the ring-closing step. The developed conditions produced good functional group tolerance with a wide range of high-profile furocoumarin product. The potential for this strategy to be applied in other syntheses of heterocyclic compounds is highly achievable.

A novel synthesis of furocoumarins was developed by a reaction between oxime esters and 4-hydroxycoumarins.

Oxidation pathways, kinetics and branching ratios of chloromethyl ethyl ether (CMEE) initiated by OH radicals and the fate of its product radical: an insight from a computational study

The OH-initiated oxidation reactions of chloromethyl ethyl ether (CH2ClOCH2CH3) have been presented by using quantum calculation methods. The Minnesota functional (M06-2X) of the density functional theory method along with a polarization and diffuse 6-311++G(d,p) basis set is chosen for optimization and frequency calculations for H-abstractions from CH2ClOCH2CH3 molecules by OH radicals. Furthermore, the CCSD(T) method along with the same basis set is used for energy refinement of all optimized structures to obtain more accurate energies of the species. Our thermo-chemical calculation results show that the C˙HClOCH2CH3 product radical is more stable, corresponding to hydrogen atom abstraction from the -CH2Cl site, than others while the energy profile results indicate that the H-atom abstracted from the -OCH2 site follows the minimum energy path compared to other channels. The rate constants are computed using canonical transition state theory (CTST) within the temperature range of 250-450 K at 1 atm. The overall rate constant (at 298 K) for the abstraction reactions is found to be consistent with the earlier reported rate constant. The percentage branching ratios of different abstraction channels and the lifetime of chloromethyl ethyl ether are also given herein. We further investigated the unimolecular decomposition pathways of the CH2ClOCH(O˙)CH3 radical and found that unimolecular C-C bond scission is the kinetically and thermodynamically more feasible pathway compared to other unimolecular decomposition reactions.

Laser Photolysis Kinetic Study of OH Radical Reactions with Methyl tert-Butyl Ether and Trimethyl Orthoformate under Conditions Relevant to Low Temperature Combustion: Measurements of Rate Coefficients and OH Recycling

Methyl tertiary butyl ether (MTBE) and trimethyl orthoformate (TMOF) are potential biofuel ethers and could replace conventional fossil fuels, or act as additives to aid combustion. Laser flash photolysis with laser-induced fluorescence detection of the OH radical has been used to measure the rate coefficients of the OH reaction with these ethers, from 298 K to approximately 740 K. The temperature dependence of the rate coefficients is parametrized as k_OH+MTBE(298-680 K) = 9.8 × 10^-13× ( T/298)^2.7 × exp(2500/R T) cm³ molecule^-1 s^-1 and k_OH+TMOF(298-744 K) = 8.0 × 10^-13 × [( T/298)^2.6 + ( T/298)^-8.1] × exp[2650/R T] cm³ molecule^-1 s^-1. The room temperature (298 K) bimolecular rate coefficients were measured as k_OH+MTBE = (2.81 ± 0.32) × 10^-12 cm³ molecule^-1 s^-1 and k_OH+TMOF = (4.65 ± 0.50) × 10^-12 cm³ molecule^-1 s^-1 where the errors represent statistical uncertainties at the 2σ level in combination with an estimated 10% systematic error. Regeneration of OH radicals was observed for both reactions at higher temperatures in the presence of O₂ via biexponential OH decays, which were observed above 489 K and 568 K, for TMOF and MTBE respectively. The OH yield from MTBE/O₂, between 620 and 700 K, was invariant with the concentration of oxygen (10¹⁵-10¹⁸ molecules cm^-3) at (36 ± 5)%. Mechanisms for OH regeneration from MTBE are briefly discussed and compared with those in the literature and from dimethyl and diethyl ether. The lower OH yield from MTBE, compared to these other ethers, is most likely due to competition with an HO₂ formation channel.

Low-Temperature Autoignition of Diethyl Ether/O2 Mixtures: Mechanistic Considerations and Kinetic Modeling

Autoignition processes are of fundamental kinetic importance as well as of practical relevance for combustion devices. In recent years, diethyl ether (DEE) has attracted increasing attention as a diesel additive and also serves as a test compound in fire-safety-related studies. In the present work, a kinetically parameterized reaction mechanism for the autoignition of DEE is developed. It consists of a DEE-specific part supplemented by a base mechanism taken from the literature that contains the C1/C2 hydrocarbon and the H2/O2 reaction systems. The complete mechanism is validated against experimental ignition delay times available from the literature for temperatures ranging from 500 to 1300 K and reactant pressures between 3 and 5 bar (T=500−900 K) and between 10 and 40 bar (T=900−1300 K). The absolute values and the temperature dependence of the ignition delay times are satisfactorily reproduced. This includes important autoignition characteristics such as one- and two-stage ignitions and the so-called negative temperature coefficient regime where ignition delay times increase with temperature. Detailed kinetic-mechanistic explanations for all these phenomena are given.

Atmospheric oxidation mechanism of OH-initiated reactions of diethyl ether – the fate of the 1-ethoxy ethoxy radical

The 1-ethoxy ethoxy radical resulting from the secondary peroxy chemistry in the oxidation of diethyl ether (DEE) by hydroxyl radical leads to the formation of ethyl formate in major quantities and ethyl acetate in minor quantities.

Formation of furan along with HO₂ during the OH-initiated oxidation of 2,5-DHF and 2,3-DHF: an experimental and computational study

Experimental characterization of products during OH-initiated oxidation of dihydrofurans (DHF) confirms the formation of furan accompanied by the formation of HO2 to be a significant channel in 2,5-DHF (21 ± 3%), whereas it is absent in 2,3-DHF. Theoretical investigations on the reaction of OH with these molecules are carried out to understand this difference. All possible channels of reaction are studied at M06-2X level with 6-311G* basis set, and the stationary points on the potential energy surface are optimized. The overall rate coefficients calculated using conventional TST with Wigner tunneling correction for 2,5-DHF and 2,3-DHF are 2.25 × 10(-11) and 4.13 × 10(-10) cm(3) molecule(-1) s(-1), respectively, in the same range as the previously determined experimental values. The branching ratios of different channels were estimated using the computed rate coefficients. The abstraction of H atom, leading to dihydrofuranyl radical, is found to be a significant probability, equally important as the addition of OH to the double bond in the case of 2,5-DHF. However, this probability is very small in the case of 2,3-DHF because the rate coefficient of the addition reaction is more than 10 times that of the abstraction reaction. This explains the conspicuous absence of furan among the products of the reaction of OH with 2,3-DHF. The calculations also indicate that the abstraction reaction, and hence furan formation, may become significant for OH-initiated oxidation of 2,3-DHF at temperatures relevant to combustion.

Kinetics and mechanism of gas-phase reaction of CF3OCH2CH3 (HFE-263) with the OH radical — a theoretical study

In the present study, the density functional method with recently developed M06 functionals has been used to study the reaction of CF3OCH2CH3 with the OH radical. All possible hydrogen abstraction and displacement reaction channels have been modeled. The minimum energy path on the respective potential energy surface and energetics were calculated at the M06-2X/6-311++G(d,p) level of theory. Two different reaction mechanisms were considered: (i) reactant and product complexes called the complex mechanism and (ii) the direct mechanism (reactant → transition state → product). Tunneling corrections were made using the Eckart unsymmetrical potential. The overall rate constant calculated by the complex mechanism (keff = 1.8 × 10−13 cm3 molecule−1 s−1) has been found to be in good agreement with the experimentally determined value (1.5 ± 0.25 × 10−13 cm3 molecule−1 s−1), while the rate constant calculated by the direct mechanism (kD = 7.6 × 10−14 cm3 molecule−1 s−1) is about two times lower than the experimental value. The theoretical studies show that hydrogen atom abstraction from the –CH2– site is the most favorable reaction pathway and the reaction involves prereactive and product complexes before leading to stable product formation.

Reactivity of Cl atom with triple-bonded molecules. An experimental and theoretical study with alcohols

The reactivities of the Cl atom with triple-bonded molecules were examined by determining the rate coefficients of reactions of four triple-bonded alcohols (TA), namely, 2-propyn-1-ol, 3-butyn-1-ol, 3-butyn-2-ol, and 2-methyl-3-butyn-2-ol, using the relative rate method, at 298 K. The rate coefficients (k) of reaction of the four alcohols with Cl vary in the range (3.5-4.3) × 10(-10) cm(3) molecule(-1) s(-1). These values imply significant contribution of the Cl reaction in the tropospheric degradation of TAs in the conditions of the marine boundary layer. A striking difference is observed in the reactivity trend of Cl from that of OH/O3. Although the reactivity of OH/O3 is lower with triple-bonded molecules, as compared to the double-bonded analogues, the reactivity of the Cl atom is similar for both. For a deeper insight, the reactions of Cl and OH with the simplest TA, 2-propyn-1-ol, are investigated theoretically. Conventional transition state theory is applied to compute the values of k, using the calculated energies at QCISD and QCISD(T) levels of theory of the optimized geometries of the reactants, transition states (TS), and the product radicals of all the possible reaction pathways at the MP2/6-311++G(d,p) level. The k values calculated at the QCISD level for Cl and the QCISD(T) level for OH reactions are found to be very close to the experimental values at 298 K. In the case of the Cl reaction, the abstraction of α-H atoms as well as the addition at the terminal and middle carbon atoms have submerged TS and the contribution of the abstraction reaction is found to be significant at room temperature, at all levels of calculations. Addition at the terminal carbon atom is prominent compared to that at the middle carbon. In contrast to the Cl reaction, only addition at the middle carbon is associated with such low lying TS in the case of OH. The individual rate coefficients of addition and abstraction of OH are lower than that of Cl. The negative temperature dependence of the computed rate coefficients in the temperature range 200-400 K shows that the difference in the TS energy of Cl and OH affects the pre-exponential factor more than the activation energy.

Thermal dissociation of ethylene glycol vinyl ether

The pyrolysis of ethylene glycol vinyl ether (EGVE), an initial product of 1,4-dioxane dissociation, was examined in a diaphragmless shock tube (DFST) using laser schlieren densitometry (LS) at 57 ± 2 and 122 ± 3 Torr over 1200-1800 K. DFST/time-of-flight mass spectrometry experiments were also performed to identify reaction products. EGVE was found to dissociate via two channels: (1) a molecular H atom transfer/C-O scission to produce C(2)H(3)OH and CH(3)CHO, and (2) a radical channel involving C-O bond fission generating ˙CH(2)CH(2)OH and ˙CH(2)CHO radicals, with the second channel being strongly dominant over the entire experimental range. A reaction mechanism was constructed for the pyrolysis of EGVE which simulates the LS profiles very well over the full experimental range. The decomposition of EGVE is clearly well into the falloff region for these conditions, and a Gorin model RRKM fit was obtained for the dominant radical channel. The results are in good agreement with the experimental data and suggest the following rate coefficient expressions: k(2,∞) = (6.71 ± 2.6) × 10(27) × T(-3.21)exp(-35512/T) s(-1); k(2)(120 Torr) = (1.23 ± 0.5) × 10(92) × T(-22.87)exp(-48 248/T) s(-1); k(2)(60 Torr) = (2.59 ± 1.0) × 10(88) × T(-21.96)exp(-46283/T) s(-1).

Rate coefficients for the reaction of OH with CF3CH2CH3 (HFC-263fb) between 200 and 400 K: ab initio, DFT, and transition state theory calculations

Rate coefficients for the reaction of the hydroxyl radical with CF(3)CH(2)CH(3) (HFC-263fb) were computed using ab initio methods, viz. MP2, G3MP2, and G3B3 theories between 200 and 400 K. Structures of the reactants in the ground state (GS) and transition state (TS) were optimized at MP2(FULL)/6-31G*, MP2(FULL)/6-311+ +G**, and B3LYP/6-31G* level of theories. Seven TSs were identified for the title reaction in the above theories. However, five out of seven TSs were found to be symmetrically distinct. The kinetic parameters due to these five different TSs are presented in this manuscript. Intrinsic reaction coordinate (IRC) calculations were performed to confirm the existence of transition states. The contributions of all the individual hydrogens in the substrate for the reaction are estimated and compared with the results obtained using Structure Additivity Relationships. The rate coefficients for the title reaction were computed to be k = (7.96 +/- 0.93) x 10(-13) exp [-(2245 +/- 30)/T] cm(3) molecule(-1) s(-1) at MP2, k = (9.50 +/- 0.93) x 10(-13) exp [-(1162 +/- 30)/T] cm(3) molecule(-1) s(-1) at G3MP2, and k = (7.01 +/- 0.88) x 10(-13) exp [-(753 +/- 35)/T] cm(3) molecule(-1) s(-1)at G3B3 theories. The theoretically computed rate coefficients are found to be in excellent agreement with the experimentally determined ones. The OH-driven atmospheric lifetimes of this compound are computed to be 132, 2.2, and 0.7 years at, MP2, G3MP2, and G3B3 level of theories, respectively.